1. Field of the Invention
This invention relates to belt retractors for vehicle occupant restraint systems and more particularly to reel type retractors which lock against rotation in a belt-extending direction under emergency conditions.
2. Prior Art
Reel type seat belt retractors which allow extension of the belt in response to a pull under normal operating conditions but which sense a sudden pull on the belt or deceleration of the vehicle as may occur in an accident to lock the reel against further belt extension provide a number of advantages over alternate forms of retractors. They allow the belt system to be used by occupants of different size without adjustment, and allow the occupant to move freely in his seat without releasing the belt or causing the belt to tighten on the occupant as webbing retraction sensitive systems do. Despite these advantages, the adoption of acceleration sensitive locking retractors has been hindered because of the lack of a relatively simple and reliable design. Several factors tend to complicate the retractors of this type and at the same time reduce their reliability. There is the antithetical design requirement that a relatively small force actuate the locking mechanism which must withstand the very high forces generated by the impact of the moving occupant's body in a sudden vehicle deceleration. Another design problem relates to the fact that the reel must be stopped while it is moving relatively rapidly and locking parts do not always meet in the same position.
One known type of emergency locking retractor employs a rotatable flywheel supported on the reel urged by a spring to move with the reel in a web extending direction. If the reel accelerates relatively slowly, as it does in normal usage, the forces required to overcome the inertia of the flywheel will not be sufficient to compress the spring and the flywheel will rotate with the reel; however, when the reel is rapidly accelerated as by the occupant's body moving forward rapidly in the seat, the inertia forces cause the flywheel to slip relative to the reel, compressing the spring. A dog is supported in a pair of complementary cavities formed in the flywheel and reel so that it moves outward radially in response to relative motion between the two and engages an internal circular ratchet surface formed on a member which is fixed to the reel base. This engagement locks the reel against further rotation. In retractors of this design, the dog must be relatively light weight so the flywheel is well balanced yet the dog must withstand the impact of its engagement with the stationary ratchet and absorb the forces which restrain the occupant's body against forward motion. Since the dog may move outwardly at any angular position of the reel, difficulty is encountered in designing the dog and ratchet to withstand the large impact forces.